This invention relates to interface circuitry which interconnects two-wire and four-wire communication channels such as telephone communication channels. More particularly, the invention is directed to a flux compensation circuit in a transformer-coupled interface circuit utilized in the supplying of direct current to a telephone or similar communication device.
In communication systems, particularly telephony, it is common practice to transmit signals between a subscriber station and a central switching office via a two-wire bidirectional communication channel. Between central offices, communication is implemented frequently by use of a four-wire communication channel comprising a pair of two-wire channels for communicating in opposite directions. An interface circuit, or SLIC connects the two-wire channel with the four-wire channel. The SLIC directs the signal transmission between the two-wire bidirectional channel and the input and the output portions of the four-wire communication channel while inhibiting any coupling of incoming and outgoing communications between the corresponding portions of the four-wire communication channel.
One form of interface circuit employs transformer coupling between a subscriber telephone line and conversion circuitry wherein the incoming and the outgoing portions of the four-wire channel connect with the bidirectional subscriber two-wire channel. Communication signals are coupled via the transformer. An example of a prior-art transformer coupling in such interface circuitry is depicted in FIG. 1 of U.S. Pat. No 4,161,633 issued in the name of R. Treiber on July 17, 1979 and assigned to the assignee hereof. The primary side of the transformer connects with the subscriber line and, in addition to the coupling of communication signals, is also employed for applying direct current from a central office battery to power a telephone on the subscriber line. The transformer circuit is convenient and often used because it provides the two functions of coupling communication signals and the direct power.
A problem arises in that transformers tend to be substantially larger than other electronic components in the interface circuit, the magnetic core of the transformer is considerably heavier than other components, and the cost of the transformer increases with increasing size of the transformer. While a relatively small transformer may be employed for the single task of communicating voice and data signals over the telephone line, the additional task of providing the direct current power to the subscriber telephone line has necessitated a larger core so as to prevent saturation of the core by the presence of the direct current. It is readily appreciated that any saturation or near saturation of the core would decrease the inductance of each winding of the transformer and reduce the coupling between primary and secondary windings. Thus, the transformer could no longer effectively couple the communication signals in the event that the direct current were to saturate the core. As a result, the transformers utilized for the two functions of coupling the signals and the power must be of significantly larger size than would be required for only the coupling of communication signals.